Boilers mainly used for supplying heating water and hot water at home can be classified into general boilers and condensing boilers according to a method of recovering combustion heat for heating water. The condensing boilers recover latent heat of water vapor involved in exhaust gas and are provided with a separate heat exchanger (latent heat exchanger) to recover latent heat.
Such a latent heat exchanger firstly absorbs combustion heat of a burner, and secondly makes contact with exhaust gas which comes into contact with a sensible heat exchanger exchanging heat with water in an inner pipe so as to have a lowered temperature. As the latent heat exchanger makes contact with relatively low temperature of the exhaust gas, condensed water is generated. Hence, the latent heat exchanger is preferably made of a material, for example, stainless steel or aluminum, etc., to resist corrosion caused by the condensed water (representing acidity due to nitrogen oxide included in the exhaust gas).
FIG. 1 is a view showing a configuration of a conventional downstream-type condensing boiler. As shown in FIG. 1, the condensing boiler for supplying heating water and hot water includes a sensible heat exchanger 11 and latent heat exchanger 12 for absorbing combustion heat of a burner 10 in order to exchange heat with water in an inner pipe, which are made of a single pipe. Both ends of the sensible and latent heat exchangers 11 and 12 are connected to a heating water supplying port 13 and a heating water returning port 14 of the heating pipe arrangement, respectively. Further, the condensing boiler is provided with an indirect heat exchanger 17 disposed between the heat water supplying port 13 and the heat water returning port 14, which transfers heat energy absorbed by heating water supplied through the heating water supplying port 13 to cold water introduced into a cold water pipe 15, so as to heat the cold water and supply the heated water, i.e. hot water, to a hot water pipe 16.
A reference numeral 18, as not described above, indicates an expansion tank for removing air created in the heating water pipe, decreasing pressure caused by the expansion of beating water in the pipe according to an increase of a temperature of heating water, and constantly maintaining the amount of water in the boiler. A reference numeral 19 denotes a blower for supplying combustion heat of the burner 10 to the sensible heat exchanger 11 and the latent heat exchanger 12. A reference numeral 20 indicates a three-way valve, a reference numeral 21 indicates a circulation pump, a reference numeral 22 denotes a condensed water drain, and a reference numeral 23 indicates an exhaust port.
In such a boiler, a three-way valve 20 is essentially installed in order to switch the flow of heating water supplied from the heating water supplying port 13 to the heating water pipe to the indirect heat exchanger 17, so that the boiler has a closed circuit for the heating water. Hence, there is a problem in that the boiler has a complicated structure.
Further, since the conventional sensible and latent heat exchangers are made from a single pipe, there is another problem in that a double pipe heat exchanger for supplying heating water and hot water cannot be used as a sensible heat exchanger in the condensing boiler.